Optical data storage media often take form in a data-carrying layer formed on a disk shaped polycarbonate substrate. These devices typically fall in the category of substrate-incident media since data recorded in the data-carrying layer is retrieved by illuminating the data-carrying layer through the substrate. More particularly, illumination light for reading the data first passes through the substrate before being received by the data-carrying layer surface.
Optical data storage media take form in one of three distinct embodiments: read-only optical data storage media, write-once optical data storage media, or rewritable optical data storage media. In read-only optical data storage media, data is recorded in the data-carrying layer as a series of physical marks or bumps. These physical marks or bumps are typically formed using an injection-molded process. Once formed the physical marks or bumps cannot be erased or overwritten. The physical marks or bumps have different diffractive properties when compared to areas of the read-only data-carrying layer that lack physical marks or bumps. Data retrieval in read-only optical data storage media is facilitated by the difference in diffractive properties.
Write-once optical data storage media commonly take form in a polycarbonate substrate spin coated with a layer of organic dye. Data is recorded by illuminating the organic dye layer through the substrate with a light beam whose intensity is modulated in accordance with data to be recorded. Select areas of the organic dye subjected to high intensity light chemically change and result in “dark” areas, i.e., areas which have a lower light reflectivity when compared to areas of the organic dye which are not subjected to high intensity light. To achieve compatibility with read-only optical data storage media, data is recorded as a sequence of low reflectivity dark regions each one of which is positioned between spaces of high reflectivity. This difference in reflectivity facilitates data retrieval. The chemical change in the organic dye is irreversible. Thus, data written to the organic dye layer cannot be overwritten.
Unlike read-only or write-once optical data storage media, data recorded in the data-carrying layer of rewritable optical data storage media can be erased and replaced with new data. The data-carrying layer of rewritable optical data storage media may take form in a phase-change material. The phase-change material is capable of phase change between amorphous and crystalline states. Initially, the phase-change material is formed on the polycarbonate substrate in an amorphous state. Thereafter, the state of the entire phase-change layer is changed to the crystalline state by illuminating the phase-change layer through the substrate with a high intensity light. Data is recorded in rewritable optical data storage media by illuminating the phase-change layer through the substrate with a light beam modulated in accordance with data to be recorded. The modulated light beam causes select areas of the phase-change layer to change from crystalline to amorphous. Crystalline regions have higher light reflectivity when compared to amorphous regions. In rewritable optical data storage media, data is recorded as a sequence of low reflectivity amorphous regions each one of which is positioned between crystalline spaces of high reflectivity. This difference in reflectivity facilitates data retrieval. Recorded data may be erased by illuminating the entire phase-change layer, or sub-regions thereof, through the substrate with a high intensity light. The high intensity light changes the amorphous areas back to the crystalline state.